In Real Life, if a school is planning a major change of Dress Code, particularly if they're going all the way from "no shirt, no shoes, no service" to a strict uniform policy, they will announce it well in advance, seeking consultation from parents, teachers, the community at large if it's a public school, and often students, with any changes to take effect at the start of the following school year. (This applies to most other major policy changes, too. Schools are run by bureaucracies, and bureaucracy tends to move ponderously.)
Not so in fictionland. Maybe the Reasonable Authority Figure has been replaced by someone less reasonable, maybe the principal just snapped, maybe it's coming down from above, but the fictional school now has uniforms. With a day's notice, a week's notice at most. The resolution of the episode will lead to them being abolished since Status Quo Is God, usually reverting to No Dress Code at all.
Often found within a Tyrant Takes the Helm plotline. Primarily seen in American series, since it needs on the one hand a serial medium, such as television, and on the other a cultural backdrop in which school uniforms exist as an option note but are the exception rather than the norm. This is something of a compressed variant of SoCalization since most middle schools in Los Angeles have uniforms but they're far less common at both the elementary and High School level. Sub-Trope of Dress Code. See also Forced into Their Sunday Best.
- Ranma ½ had something similar with principal Kuno introducing a mandatory hairstyle (or trying to) in addition to the existing uniforms.
- Archie Comics had a story arc revolving around this.
- Invoked as a minor running gag in Harry Potter and the Order of the Phoenix, where the first thing we see Dolores Umbridge do after being appointed Headmistress of Hogwarts is her marching around the corridors magically rearranging all the pupil's uniforms until they are up to military standard.
- Averted with the Eric Walters book Branded, where school uniforms are announced early in the year after the bureaucracy has already decided on them and are phased in over a couple of months.
- iCarly had this when Principal Franklin was kicked out.
- Degrassi played this straight with the implementation of uniforms over Christmas break without prior discussion, but subverted it by keeping the new dress code in place for a year in-story (half the 10th and the entire 11th season).
- They also did an episode centered around kids having to still wear parts of it because their families couldn't afford to replace serviceable clothes.
- Little House on the Prairie did this when Mrs. Oleson took over the school. It's possibly the earliest example and memorable because the school board ignored a glaring conflict of interest in the part-owner of the only clothing and dry-goods store in town, forcing parents to buy additional clothing.
- The series of 10 Things I Hate About You.
- Shake It Up!, "Protest it Up". CeCe lets out a Big "NO!" in response, but Gunther and Tinka have great amounts of extra time and energy when they're not planning out over-the-top outfits on a daily basis...
- Not a "school" uniform as such, but the Star Trek: The Next Generation episode "The Chain of Command" has got this trope in all other ways. A new guy comes in and replaces Picard as the captain of the Enterprise, and one of the things he changes immediately is in ordering Counsellor Troi to start wearing her duty uniform (Troi had spent the previous six years wearing a variety of informal style leotards and dresses). It's one of a number of things designed to show us that the new captain is a very different guy to Picard.
- In the TV adaptation of Clueless, the season one episode "The Party's Over" sees the school gain a new principal, who is disgusted with what he saw as the students' lack of discipline. His decision is to institute uniforms overnight, leading to Cher having imaging the school becoming a place of robotic clones. The episode ends with Cher reworking the "I Have A Dream" speech into a speech about individual freedom with the principal conceding he was wrong.
- In How To Rock the principal does this in response to Kacey and Molly's attempts to out-over-the-top each other leading to his being physically injured by their hats. He specifically called out Kacey and Molly for bringing it down on the whole school and when he relents at the end of the episode the two of them still have to wear it.
- In Wizards of Waverly Place this happens as an act of enforced discipline that was misdirected.
- That's So Raven had an episode. Raven organizes a protest (everyone is supposed to technically wear the uniform, but in a highly personalized way), but everyone backs out except the mean clique. The plot of the episode is more about Raven accidentally falling in with them, but it does follow the trope.
- Andi Mack did a sidelong glance off this trope when the new principal starting to enforce an already-existing but long-ignored dress code, with absolutely zero prior warning.
- This routine happened in El Goonish Shive. Principal Verruckt of Moperville North High School decided to have all of the students wear uniforms (apparently without consulting with parents, the local school board or anyone else) and put the policy into effect almost immediately. It's also a splendid example of Webcomic Time, since the policy was in place for less than a week In-Universe but covered 3½ years' worth of strips (most of which was taken by the birthday party arc).
- Danny Phantom: Vlad Masters, the newly elected mayor, makes this a rule at the local high school, specifically to annoy Perky Goth Sam, since she's a close friend/sidekick to Vlad's Arch-Enemy, Danny.
- My Gym Partner's a Monkey had the episode "Uniformity", where Charles Darwin Middle School instates a prep-school uniform that causes everyone to act uncharacteristically genteel and proper (read: boring), in stark contrast to the students' usual antics. The uniform was instated in about a week and later revoked in less than a day.
- Pepper Ann once deconstructed this. While the school ends up relenting the uniforms at the end of the episode, the students think the uniforms are cute, and still wear them casually in the end.
- The Simpsons. The school uniforms were abandoned when the gray dyes ran to a tie-dye look in the rain.
- Averted in Disney's Doug. The plan is announced at the beginning of the episode. Over the course of the episode, the student body collects signatures on a petition, forms an interest group, sets up protests, and eventually splits based on difference of opinion. By the end of the episode, the student body has accepted the imposition of school uniforms, but because the adults can't decide on a design, the plan is put on hold until the next year.
- The Kids From Room 402: Mr. Besser once made the students wear uniforms. The decision was short-lived because the uniforms prevented him from pinpointing culprits whenever somebody broke a rule.